Boats which plane on the surface of water with one or more boat-propelling machines generally take, in a non-planing state or prior to start of planing on the surface of water, a forwardly-and-upwardly inclined posture or position with the bow raised and the stern lowered and partly submerged under water; the boats start planing in such an inclined position. At the beginning of planing on the surface of water, a sufficient boat speed can hardly be obtained due to a great water resistance.
In order to secure certain speeds, it is necessary that the boat body (i.e., hull) take a substantial horizontal posture with the stern raised to a certain degree; however, it would take a considerably long time for the boat hull to assume such a substantial horizontal posture, so that smooth acceleration can not be readily achieved.
Thus, lift force generation devices for lifting upward the stern and boat-propelling machine at the beginning of planing travel are proposed, for example, in Japanese Patent Laid-Open Publication Nos. SHO-57-60995 and SHO-59-130799 (hereinafter “Patent Document 1” and “Patent Document 2”, respectively).
The lift force generation device disclosed in Patent Document 1 includes a cavitation plate and splash plate mounted on a portion of a lower casing of the boat-propelling machine above a propeller, and an acceleration plate mounted above the cavitation plate and splash plate. The acceleration plate is in the form of a flat plate that projects leftward and rightward and forward and rearward from the lower casing, with the forward projecting amount of the acceleration plate being smaller than the rearward projecting amount.
Further, the right force generation device disclosed in Patent Document 2 includes a cavitation-preventing plate mounted above the propeller of the boat-propelling machine, and a buoyancy plate of a wing-like sectional shape mounted above the cavitation-preventing plate.
In each of the lift force generation devices disclosed in Patent Document 1 and Patent Document 2, the acceleration plate or buoyancy plate is fixed to a projecting section of a body part of the boat-propelling machine via stays or bolted to threaded portions of the body above the propeller; namely, the acceleration plate or buoyancy plate is fixed via a local mounting structure.
Where the acceleration plate or buoyancy plate is unnecessary and thus not mounted on the body of the boat-propelling machine, the mounting section for the acceleration plate or buoyancy plate is exposed on the body, so that the outer appearance of the boat-propelling machine would be aesthetically impaired.
Further, the lift force generation device disclosed in Patent Document 1 would encounter a structural limitation of a mounting bracket. In any case, because the large body of the boat-propelling machine has the dedicated mounting structure, there would arise inconveniences, such as high manufacturing cost of the body part.
Furthermore, because the acceleration plate disclosed in Patent Document 1 is in the form of a flat plate that does not greatly extend rearward up to or beyond the rear end of the propeller, the boat's stern normally partly submerged under water can not be lifted up quickly and smoothly at the beginning of planning operation of the boat-propelling machine. As a consequence, it would take a long time for the boat's hull to assume a substantial horizontal posture, so that smooth acceleration tends to be difficult to achieve.
Furthermore, the buoyancy plate of a wing-like sectional shape, disclosed in Patent Document 2, has a smaller length in the front-rear direction than the cavitation-preventing plate and has its rear end located forward of the rear end of the cavitation-preventing plate. Thus, as with the acceleration plate disclosed in Patent Document 1, the boat's stern normally partly submerged under water can not be lifted up quickly and smoothly at the beginning of planning operation of the boat-propelling machine. As a consequence, it would take a long time for the boat's hull to assume a substantial horizontal posture, so that smooth acceleration tends to be difficult to achieve.